Yep, I can see that /dev/sda failed. Looks like something also happened to /dev/sdb too, but I'm not entirely sure what. The array SHOULD have come up in degraded mode with 3 of the 4 disks.

So let's try bringing it up in degraded mode:

Code:

mdadm --assemble --run --force /dev/md0 /dev/sdb4 /dev/sdc4 /dev/sdd4

If that works it'll print something like: "mdadm: /dev/md0 has been started with 3 drives". Move on to:

Code:

mount -o ro /dev/md0 /mnt

This will mount the degraded array read-only and let you start moving files off it onto some other media. If you had a trustworthy /dev/sda then I'd consider rebuilding the array, but that will come later after you get a fresh drive back from WD.

mount: wrong fs type, bad option, bad superblock on /dev/md0,
missing codepage or helper program, or other error
In some cases useful info is found in syslog - try
dmesg | tail or so
I'm pretty sure the filetype should be ext2 but I tried that and it didn't work. It didn't display anything either

mount: wrong fs type, bad option, bad superblock on /dev/md0,
missing codepage or helper program, or other error
In some cases useful info is found in syslog - try
dmesg | tail or so
I'm pretty sure the filetype should be ext2 but I tried that and it didn't work. It didn't display anything either

OK, as I suspected the filesystem itself is damaged. In addition to a drive being missing. So we'll have to try to recover it.

Run:

Code:

testdisk /dev/md0

(If TestDisk isn't installed, you'll have to either install it, or find a Live CD which includes it, as I originally suggested.)

Choose Proceed on the first screen. On the partition type screen choose None. On the next screen, go down to Advanced and choose that. Then choose Superblock. At that point it will start scanning the array for superblock backups. This could take a few minutes. The output should look something like this: